492 Produce Degradation: Reaction Pathways and their Prevention
Tn 5 into a cluster of genes in the Type II secretion gene family [103,104], were
unable to induce soft rot on potato tuber slices or bell pepper fruits [71]. The synthesis
and secretion of PL thus represent two consecutive functions required by P. fluore-
scens and P. viridiflava to be efficient soft-rotting pathogens.
Production of PL in certain strains of P. fluorescens is induced by pectic sub-
strates [53,105] or by plant tissue extracts [106–108]. However, in other P. fluore-
scens strains, production of PL is not affected by the type of carbon source included
in the medium [53,108]. Recently, we investigated the mode of PL production in 24
strains of P. fluorescens and found that production of PL in 4 out of 24 strains was
not induced by pectic substrates but by Ca+2 [109]. These four strains produce 10
times more PL in medium containing 1 mM CaCl 2 than in one containing no CaCl 2
supplement. Presence of CaCl 2 in the medium not only affects the amount but also
the final destination of PL. Over 86% of total PL produced by strain CY091 in
CaCl 2 -supplemented medium was excreted into the culture fluid. By comparison,
only 13% of total PL produced by this strain in CaCl 2 -deficient medium was detected
in the extracellular fraction. The effect of Ca+2 on PL (and also Prt) production is
concentration-dependent and can be replaced by Sr+2, but not by Zn+2, Fe+2, Mn+2,
Mg+2, or Ba+2. Because of the indispensable role of Ca+2 in PL production and pectic
degradation, the potential of using ion-chelating agents such as EDTA for control
of Pseudomonas rot has been investigated [109]. Treatment of potato tuber disks
with 40 ppm of EDTA especially in the presence of nisin (a bacteriocin) is effective
in suppressing the development of soft rot [109,110].
16.5 INTERACTIONS OF PF PSEUDOMONADS AND
NATIVE MICROFLORA ON FRESH PRODUCEThe changes in microflora on fresh and fresh-cut produce as affected by processing,
decontamination treatments, and storage conditions have been extensively investi-
gated and reviewed [5,6]. The numbers and the types of microorganisms identified
are variable and largely dependent on the sources of the samples analyzed. The
population of mesophilic bacteria as determined on plate count agar can range from
103 to 10^9 colony-forming units (CFU) per gram of tissue. Very diverse groups of
microflora are present on the surfaces of fresh fruits and vegetables. In addition to
fluorescent pseudomonads and Erwinia, other genera of microflora including Ser-
ratia, Klebsiella, Citrobacter, Enterobacte, yeast, and LAB have been detected on
various types of produce [17,18–20,22,40,42,43,45,47,48]. PF pseudomonads often
constitute a major proportion of native flora on salad vegetables [17], shredded
lettuce [45], cauliflower florets [47], endive leaves [22], spinach [42], tomatoes [41],
and alfalfa seed [25], and appear to play the critical role in the development of soft
rot or spoilage. As discussed above, softening and maceration of plant tissues results
mainly from the action of PL or other depolymerases produced by pectolytic Erwinia
and fluorescent pseudomonads.
The coliforms and enterobacteria present on the surfaces of fresh produce are
generally considered saprophytic and nonpectolytic, although production of PL and
exoPG within the cells has been detected in certain strains of Klebsiella and Yersinia
[115,116]. The role of these enterobacteria in spoilage of fresh produce is unclear,